Interplanetary Fall

Interplanetary Fall

Today our planet joins two other worlds in the solar
system where it is northern autumn.

Won't be long 'til
summer time is through...
The Beach Boys, "All
Summer Long"

September
22, 2000 -- Every year around this time thousands of penguins
rejoice to see the Sun peep above the Antarctic horizon. The
return of sunlight after nearly 6 months of chilly darkness means
it's time to shed a few pounds of blubber, find a mate, and bask
in the sunshine. Spring is in the air.

On the other end of the world, sun-dappled puddles of water
are freezing at the north pole, while Arctic bears are pondering
hibernation. The north polar Sun, circling downward on a horizon-skirting
360 degree spiral, will soon be gone.

What powerful force of nature can rouse the passions of penguins
at one end of the world and put mighty bears to sleep at the
other? It's the changing of the seasons from northern summer
to fall-- a special date on the calendar that northerners call
the autumnal
equinox.

Above: This is an image of the Automated Astrophysical
Site-Testing Observatory near the US South Pole Station captured
less than one day before the 2000 September equinox. The soft-yellow
dawn sky presages the coming of a 6 month-long day. Click
for images updated every 10 minutes.

For the past six months our planet's north pole has been tilted
toward the Sun, most directly on June 21st, which was the beginning
of northern summer. But, as The Beach Boys pointed out in a popular
tune, there's no such thing as an Endless Summer. Today
(Sept. 22nd) at 1727 UT (1:27 pm EDT) our planet's "subsolar
point" crossed the equator heading south. With it, spring
began in the southern hemisphere and autumn in the north.

On days when the Sun is shining
directly over the equator, daylight and darkness are of nearly
equal length. The word equinox comes from a Latin word
meaning "equal night."

Left: The red dot marks Earth's subsolar point (the
location where the Sun is directly overhead) at noon Eastern
Standard Time throughout the year. Equinoxes occur when the subsolar
point crosses the equator, once in March (the Vernal Equinox)
and again in September (the Autumnal Equinox). This animation
is based on images generated by JPL's
Solar System Simulator.

Contrary to the all-too-popular notion that Earth is closer to
the Sun during summer and farther away during winter, seasons
are not caused by the eccentricity of our planet's orbit.
Indeed, during the hottest days of northern summer the Earth
is at its greatest distance
from the Sun.

The primary cause of seasonal extremes on Earth is the 23
degree tilt of our planet's spin axis. When the north pole is
tilted toward the Sun, northern days are long and the weather
is warm. Six months later, as the south pole tilts toward the
Sun, the southern hemisphere takes its turn at summer.

Seasons in the two hemispheres are always reversed. When it
is summer in New York, it is winter in Sydney. On a spring day
in Paris, autumn leaves are falling in Argentina.

With the coming of fall in Earth's northern hemisphere,
our planet joins two other worlds where it is also northern autumn:
Saturn and Neptune. Fall on those planets isn't much like Earth.
There are no golden leaves that fall to the ground or gently
chilled breezes that hint of the coming winter. Saturn and Neptune
don't even have a "ground" for leaves to land on. Both
are giant gaseous balls of hydrogen and helium. The "gentle"
chill of 1000 km-per-hour autumnal winds on Saturn registers
-130 C. That's balmy compared to Neptune, where temperatures
are 100 degrees lower and the winds blow nearly as hard.

While seasons on other planets may seem alien, they are defined
just as they are on Earth. When the Sun shines down directly
over a planet's equator -- that's an equinox. When one of the
poles is tilted toward the Sun to its maximum extent -- that's
a solstice. The equinoxes and solstices for 8 of the 9 planets
are tabulated below. (Pluto is omitted, because we know so little
about that distant world.)

The unfamiliar seasons of other worlds would strike any Earth-dweller
as strange -- but some planets are more alien than others.

Take Mars for example. Mars has the highest orbital eccentricity
of any planet except Mercury and Pluto. Its distance from the
Sun varies between 1.64 and 1.36 AU over the Martian year. This
large variation, combined with an axial tilt greater than Earth's,
gives rise to a very peculiar seasonal change. When Mars is closest
to the Sun, it is wintertime at the planet's north pole. Bone-chilling
temperatures at that end of the planet plunge so low that carbon
dioxide -- the main constituent of Mars's atmosphere -- freezes
and falls to the ground. So much of the red planet's atmosphere
turns to ice, that the global atmospheric pressure drops by 25%!
Mars's atmosphere is noticeably thinner during northern winter.

Martian seasons are peculiar by Earth-standards, but they pale
in comparison to seasons on Uranus. Like Earth, Uranus follows
an orbit that is nearly circular; it keeps the same distance
from the Sun throughout its long year. But, Uranus's spin axis
is tilted by a whopping 82 degrees! This gives rise to extreme
20-year-long seasons and unusual weather. For nearly a quarter
of the Uranian year (equal to 84 Earth years), the Sun shines
directly over each pole, leaving the other half of the planet
enveloped by darkness.

Left: This dramatic time-lapse
movie captured by NASA's Hubble Space Telescope shows seasonal
changes on Uranus. Once considered one of the blander-looking
planets, Uranus is now revealed as a dynamic world with the brightest
clouds in the outer Solar System. more
info.

The northern hemisphere of Uranus is just now coming out of the
grip of its decades-long winter. Sunlight, reaching some latitudes
for the first time in years, warms the atmosphere and triggers
gigantic springtime storms comparable in size to North America
with temperatures of 300 degrees below zero. In the animation
pictured left, bright clouds are probably made of crystals of
methane, which condense as warm bubbles of gas percolate upwards
through the atmosphere.

Mercury's seasons -- if they can be called that -- may be the
most remarkable of all. Mercury rotates three times on its spin
axis for each two orbits around the Sun. It is the only one of
our solar system's planets or moons tidally
locked in an orbital-to-rotational resonance with a ratio
other than 1:1.

Mercury's weird rotation combined with the high eccentricity
of its orbit would produce some very strange effects for an observer
on that planet. At some longitudes, Mercurian skywatchers would
see the Sun rise and gradually increase in apparent size as it
slowly moved toward the zenith (i.e., the solstice!). At that
point the Sun would stop, briefly reverse course, and stop again
before resuming its path toward the horizon, all the while decreasing
in apparent size. Observers at other points on Mercury's surface
would see different but equally bizarre motions.

Temperature variations on Mercury, ranging from -200 C at night
(winter) to 400 C during the day (summer), are the most extreme
of any world we know.

Parents and Educators: Please visit
Thursday's
Classroom for lesson plans and activities related to this
story.

Summer on Mercury is hot, but it's not the hottest spot in
the solar system. The surface of Venus, warmed by a runaway greenhouse
effect in Venus's carbon dioxide atmosphere, is even hotter at
470 C -- easily hot enough to melt lead.
Because of Venus's thick cloud cover and circular orbit, seasonal
changes in temperature are likely to be small. Indeed, of all
the worlds in the solar system, ever-hot Venus might be the one
with a truly Endless Summer. On the other hand, its choking
acid-rich atmosphere and perpetual cloud cover would probably
dampen enthusiasm for sun bathing.

There's no doubt that extraterrestrial seasons are harsh and
uncomfortable. So, while you're lamenting the prospect of raking
autumn leaves weekend after weekend for the next two months,
take a quick mental tour of the solar system. Yard work on Earth
might not seem so bad after all!

Web LinksEarth's
Seasons -- A table
of solstices and equinoxes from the US Naval Observatory